1. Field of the Invention
This invention relates to thermosetting resin compositions useful as underfill sealants for mounting to a circuit board semiconductor chips or semiconductor device packages, which have a semiconductor chip on a carrier substrate. Reaction products of these compositions demonstrate improved adhesion after exposure to elevated temperature conditions, improved resistance to moisture absorption and improved resistance to stress cracking.
2. Brief Description of Related Technology
In recent years, the popularity of smaller-sized electronic appliances has made desirable size reduction of semiconductor devices. As a result, chip packages are becoming reduced in size to substantially that of the bare die themselves. Such smaller-sized chip packages improve the characteristics of the microelectronic device in which it is used, while retaining many beneficial operating features. This serves to protect semiconductor bare chips, and increases their reliability and useful life.
Ordinarily, chip assemblies are connected to electrical conductors on a circuit board by use of solder connection or the like. However, when the resulting chip/circuit board structure is subjected to conditions of thermal cycling, reliability becomes suspect due to fatigue of the solder connection between the circuit board and the chip assembly. Recent manufacturing advances provide a sealing resin (often referred to as underfill sealant) in the space created by the mounting of a direct chip attach package (“DCA”) [such as a chip scale package (“CSP”)/ball grid array (“BGA”)/land grid array (“LGA”) assembly or a flip chip (“FC”) assembly] onto a circuit board to relieve stresses caused by thermal cycling. Underfill sealants have been seen to improve heat shock properties and enhance the reliability of such structures.
Of course, curable resin compositions generally are known. See e.g., U.S. Pat. No. 4,645,803 (Kohli), which relates to curable epoxy resin compositions of reinforcing filaments and epoxy resins together with a primary amine-functional curing agent with or without a polyamine curing agent and a curing catalyst which when cured into a fiber matrix is useful in preparing composites and prepreg materials for structural applications.
In addition, U.S. Pat. No. 4,499,245 (Ikeguchi) relates to a curable resin composition requiring a mixture and/or a reaction product of (a) a polyfunctional dicyandiamide, prepolymer of the dicyandiamide or coprepolymer of the dicyandiamide and an amine and (b) a polyhydantion resin—a phenolic-based epoxy curative. In addition, a polyfunctional maleimide, prepolymer of the maleimide or coprepolymer of the maleimide and an amine may be included as a component (c). These compositions are reported to be useful as coating materials for rust prevention, flame resistance, flame retardation; electrical insulation varnish; and laminates for use with furniture, building materials, and sheathing materials.
More specifically, thermosetting compositions of dicyandiamides and epoxy resins are also generally known. See e.g., Japanese patent document JP 62-275,123, an English-language abstract of which speaks to a resin composition for preparing prepreg materials with reinforcing fiber for structural applications. The compositions are reported to include certain dicyandiamides, bismaleimide, polyether sulfone (as a non-reactive thermoplast whose use is as a toughening agent) and bisphenol F- or A-type epoxy resin. In addition, the composition is reported to be optionally hardened by a hardening catalyst, one of which is noted as imidazole.
U.S. Pat. No. 4,918,157 (Gardner) relates to the use of urea compounds as latent cure accelerators for dicyandiamides, and to thermosetting dicyandiamide formulations of dicyandiamides and urea compounds. More specifically, the '157 patent claims a thermosetting composition of a dicyandiamide; a urea compound selected from alkyl aryl ureas, aryl ureas and mixtures thereof; and an epoxy resin. The curable dicyandiamide formulations of the '157 patent are reportedly useful as matrix resins, and for the production of prepreg, fiber-reinforced laminates, composites and the like.
Epoxy curing systems are also known. See e.g., U.S. Pat. No. 3,862,260 (Sellers), in which a curing agent of a trifunctional hardener (such as the reaction product of one mole of bisphenol A with one mole of formaldehyde) and an imidazole is disclosed.
These uses for thermosetting resin compositions appear to be directed to structural applications, as contrasted to the microelectronic application to which the compositions of the present invention are directed. To that end, the use of epoxy resin compositions as matrix compositions for fiber reinforcement in prepreg, composite and laminate materials for structural materials differs significantly from the use of epoxy resin compositions as an adhesive and encapsulant in microelectronic applications, such as with electrical solder junctions in semiconductor chips, and creates different demands and requirements from the uncured resin as well as cured reaction products thereof.
A drawback to resin compositions presently used in microelectronics applications, such as for underfill sealants, has been the ability of providing such resins with a commercially acceptable useful working life at room temperature or dispensing temperatures.
Generally, at temperatures near room temperature, the resins begin to cure upon introduction of the curing agent or catalyst. This causes viscosity increases which leads to reduced dispensability. While such viscosity increase may be alleviated to some degree by using a liquid curing agent or catalyst, liquid catalysts tend to decrease latency to a point which is not commercially practical with current production demands. And introduction of a solid catalyst, such as imidazole, has limited applicability because its presence often changes the rheological properties of the composition, and decreases flow.
Thus, due at least in part to their limited useful working life, manufacturing capacity of certain microelectronic production lines has been hampered by these shortcomings of conventional underfill sealant compositions.
Accordingly, it would be desirable for an underfilling sealant composition to provide good adhesive properties while flowing and curing in a reasonable time to be commercially appealing and possessing an extended useful working life.